HELPFUL INFORMATION
Debugging already done:Seems like the problem is caused due to the way startCollapsed is handled in the feature's collectData method:
src/grid/feature/Grouping.js, line 878:

Code:

if (me.startCollapsed) {
// If we start collapse, we'll set the state of the groups here
// and unset the flag so any subsequent expand/collapse is
// managed by the feature
me.startCollapsed = false;
collapseGroups = true;
}

The grid's view calls this method before the store actually loaded the data. After the store finished loading the data, startCollapsed is already set to false and therefore the groups are rendered expanded.

The same problem occurs if "startCollapsed: true" is added to the "Grouping with Remote Summary" example

Possible workaround

I've found that just commenting out the flag reset in an override fixes everything and from my very brief testing doesn't break any functionality. However, I didn't delve too deep down the rabbit hole.